Method for the local hardening of railway wheelsets

ABSTRACT

The object of the invention is increasing the service life or service performance of railway wheelsets. Said technical effect is achieved by the means that, in the method for increasing the service life or service performance of railway wheelsets, in which each wheel is implemented in the form of a cylindrical working part having a running or rolling surface and a rim connected thereto by a radius transition, the metal hardening of the wheelset or of a wheel is carried out by conduction of current through rotating contact rollers, which are pressed under pressure onto the surface to be treated, wherein the heating and hardening of the surface is performed in the form of one or more strips. The invention is characterized in that the heating for the hardening can be carried out in two annular sections, or annular zones, wherein the boundary of the first section begins at the connecting line of the radius transition at the beginning of the rolling surface, and may extend across the entire width of the rolling surface. Preferably, the first annular section extends over a distance of approximately 30 mm across the width of the rolling surface. The boundary of the second section begins at the connecting line of the radius transition at the adjacent rim face and preferably extends up to 35 mm over the width of the rim surface. The local heating in each of the said sections is carried out in the form of one or more annular strips, wherein the heating temperature exceeds the temperature of the phase transition Ac 3  by approximately 60-280° C. If the heating is carried out along multiple strips, the distances between the sections of the heat action of each strip are preferably at least approximately 2-4 mm from each side of the heating zone. The rotational speed of the wheelset during heating is preferably approximately 3.5-15 rotations per hour.

FIELD OF THE INVENTION

The invention concerns the field of railway vehicle operation and, in particular, a method for the local hardening of railway wheelsets or other steel wheels.

BACKGROUND OF THE INVENTION

In the operation of rail-borne vehicles, a complex and intensive interaction takes place between wheelsets and the working surface of the rails.

As a result of a complex interplay of operating factors, increased metal wear occurs in the zones of greatest contact stress between the wheelsets and the rails, resulting in premature rejection of wheel pairs. These zones are also called premature wear zones. Depending on the operating conditions, metal wear occurs primarily on the wheel flange surface and on the rolling surface.

In accordance with operating safety requirements for wheelsets, wheelsets must be repaired when the wear on the wheel rim exceeds 24 and 28 mm for freight cars and passenger cars. A maximum of four repairs in all are permissible, after which the wheelsets are unusable and must be disposed of. For the most part, the performance of repair work leads to downtime of the rail-borne vehicle, and requires great expenditures of money and time.

From the foregoing, it follows that increasing the service performance or service life of railway wheelsets represents an extremely timely concern.

From RU 2153008 is known a method for local hardening or strengthening of railway wheelsets that provides for heating of the wheel's working surface by means of conduction of current through movable electrodes that are pressed under pressure onto the surface to be treated, and with their aid heat-treated areas in the form of strips are produced on the working surface of the wheel.

The disadvantages of this method are as follows:

-   1. The impossibility of obtaining wheel pairs having the same     characteristics in the hardened zones, since the technologies of     their heat treatment are not specified. In the case of significant     overheating of the metal above the Ac₃ point (In the steel heating     process, the Ac₃ point determines the temperature of the     transformation of the pearlite structure to the austenite structure.     This temperature depends upon the chemical composition of the steel,     and can fluctuate between 727 and 911° C. For hardening, the steel     details are heated to temperatures of 20-40° C. above the Ac₃     point.), the local zones of structural transformation will be large,     which will result in high residual stresses in the details.     Moreover, the wheelsets can have a high degree of hardness while at     the same time having low plasticity, which can result in brittle     fractures of the hardened areas during the course of operation. In     the case of underheating below the Ac₃ point, the desired hardness     of the metal, and hence the promised effect, is not achieved. -   2. The ratio of hardness between the working surfaces of the     wheelsets and the rails is not taken into account. Significantly     excessive hardness of the wheelsets can lead to intensive premature     wear of the working surface of the rails. Such a situation can be     associated with great material losses. -   3. Not mentioned are the areas of local heat action and their     parameters, which determine the properties of the treated zones and     consequently the operating life of the wheels.

In addition, from RU 2153007 is known a method for local hardening of railway wheelsets in which each wheel is implemented in the form of a cylindrical working surface and a rim that is connected thereto by a radius transition, comprising the hardening of the metal by conduction of current through the rotating contact roller, which is pressed onto the surface to be treated, wherein the surface is hardened in the form of strips.

The disadvantages of this method are as follows:

-   1. With local heating to standard temperatures for hardening (other     temperatures are not specified), great heat dissipation occurs into     the mass of the wheelset. For this reason, the phase transitions     from the pearlite structure to the austenite structure, with their     subsequent transition to a tempered martensite structure, do not     take place completely in the metal mass being heated, which is     located beneath the electrode, resulting in an insignificant     increase in local hardness. The cooling technologies likewise are     not specified, but the type of structure obtained, which determines     the hardness level, depends thereon. -   2. In this method, the specific locations where the local heat     treatment to improve the service performance of the wheelsets is to     be carried out are not specified. It can also occur that the strips     of hardened metal lie on the radius transition, specifically between     the rim and the rolling zone, where the stresses during operation     are greatest. Or this zone is exposed to softening under the     influence of temperatures in the range of 650-750° C. In both cases,     the reliability and service life or service performance of the     wheelsets are degraded considerably.

Accordingly, the use of the aforementioned method will not facilitate the manufacture of railway wheelsets having a high and consistent quality that ensures increased service life or service performance.

The object of the invention is consequently to increase the service life or service performance of railway wheelsets.

SUMMARY OF THE INVENTION

This object is attained by the method according to claim 1.

The object is achieved by the means that, in the method for increasing the service life or service performance of railway wheelsets, in which each wheel is implemented in the form of a cylindrical working part having a running or rolling surface and a rim connected thereto by a radius transition, the metal hardening of the wheelset or of a wheel is carried out by conduction of current through rotating contact rollers, which are pressed under pressure onto the surface to be treated, wherein the heating and hardening of the surface is performed in the form of one or more strips. The invention is characterized in that the heating for the hardening can be carried out in two annular sections, or annular zones, wherein the boundary of the first section begins at the connecting line of the radius transition at the beginning of the rolling surface, and may extend across the entire width of the rolling surface. Preferably, the first annular section extends over a distance of approximately 30 mm across the width of the rolling surface. The boundary of the second section begins at the connecting line of the radius transition at the adjacent rim face and preferably extends up to 35 mm over the width of the rim surface. The local heating in each of the said sections is carried out in the form of one or more annular strips, wherein the heating temperature exceeds the temperature of the phase transition Ac₃ by approximately 60-280° C. If the heating is carried out along multiple strips, the distances between the sections of the heat action of each strip are preferably at least approximately 2-4 mm from each side of the heating zone. The rotational speed of the wheelset at heating is preferably approximately 3.5-15 rotations per hour.

The strips of heating can be implemented as continuous (uninterrupted) or discontinous annular sections of different configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic detail view of the interaction between a railway wheel of a wheelset and a railway rail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Analysis of the premature wear zones (zones of greatest contact stresses between the wheelsets and rails) has shown that in wheelsets these zones are located in two annular sections. In this regard, the boundary of a first section begins at the connecting line of the radius transition at the beginning of the rolling surface, and may extend across the entire width of the rolling surface. The boundary of the second section begins at the connecting line of the radius transition at the adjacent rim face and extends over a distance of up to 35 mm across the width of the rim surface. These zones of greatest stress are labeled with reference number 4 in FIG. 1. With regard to the working surface of the rails (labeled with reference number 1 in FIG. 1), the area on the outside of the radius transition of the rail head has the greatest local hardening, as is represented by the region 2 in FIG. 1. This can be explained by the fact that, as a result of the taper on the rolling surface of the rail head, a local surface hardening of the rail metal up to 800 HB takes place over a depth of approximately 0.15-0.20 mm, wherein HB indicates Brinell hardness.

Under the influence of the angular stress of the wheel pair or of the rail, a plastic displacement of the hardened upper layer of the rolling surface of the rail towards the rail head takes place, which is to say into the zone of interaction with the rim of the wheelset or the premature wear zone (zone of greatest contact stresses). The hardness of the wheelset's material, which is labeled with reference number 3 in FIG. 1, is 280 HB. As a result of the stress condition that has arisen and of the increase in the local strength in the metal of the rail head, increased local wear of the metal of the wheel takes place. The areas of increased wear are the rim zone adjacent to the radius transition, and the zone of the rolling surface adjacent to the radius transition. The increase in the local hardness of the metal in the said wheel zones to 550-800 HB makes it possible to increase the service life or service performance of the wheelsets, with the statistical average wear of the rails remaining unchanged.

The inventive method can be implemented as follows. The wheelsets are placed on a special apparatus that is equipped with a rotation mechanism and a device for concentrated local heating. The apparatus is equipped with sensors that record the parameters of the distribution of the local temperature fields and the cooling rates. The apparatus can optionally also be augmented with an additional heat source that permits thermal tempering of the treated zone as needed.

The concentrated local heating can be accomplished by means of, e.g., a special electrode. The electrode can be designed in the form of a flat, rotating roller, for example, which is pressed onto a certain point of the surface of the wheelset at a pre-determined stress.

When the apparatus is turned on, the wheel pair begins to rotate at a preferred speed of approximately 3.5 to 15 rotations per hour. The specific rotational speed is chosen as a function of the diameter of the wheelset, the width of the electrodes' working surface, the heating parameters, and the like.

Heating for the hardening preferably is carried out on all sections of the wheelset that are to be heat-treated at the same time. When this is a section on the rim, an edge of this zone or section will begin at the start of the rim at the connecting line of the radius transition. This zone is approximately 35 mm wide. When this section is located on the cylindrical part of the rolling surface, an edge of this zone starts at the connecting line of the radius transition to the adjacent rolling surface. This zone may extend over the entire width of the rolling surface. Preferably, the width of this zone can be approximately 30 mm.

In the said zones, the hardening is carried out in the form of one or two annular sections, and, specifically, preferably on the rim surface and on multiple annular sections, which may be distributed over the entire rolling surface of the wheelset.

The connecting lines of the edge of the rolling surface and of the rim to the radius transition are determined by means of a universal template for monitoring the parameters of the rolling surface of the wheelsets. This template is widely used in detecting wear and vertical thinning of the wheel flange of the rims of wheelsets.

The heating that is performed in the zone of the contact point of the electrode and the specified wheel surface takes place in the form of one or more concentric strips or annular strips. The temperature exceeds the beginning of the phase transition Ac₃ by approximately 60-280° C., and the rotational speed of the wheelset is approximately 3.5-15 rotations per hour. At this temperature and at the specified rotational speed, the phase transitions take place in the full specified region. They ensure the presence of the martensitic structure, which is distinguished by great hardness with values of approximately 550-800 HB, in the entire specified region. The depth of the section with such a structure is approximately 3-5 mm, which makes it possible to increase the resource availability of the railway wheelsets by approximately 2.5-fold.

In the region of the materials that were not heated to the temperatures of the phase transitions, the hardness of the metal remains at the starting level (280 HB). This circumstance plays an important role in ensuring the high reliability and safe operation of the railway wheelsets that have undergone local strengthening. The malleable matrix of the metal will prevent a possible occurrence and widening of cracks. As needed, the remaining thermal stresses can be eliminated in a subsequent local heating by means of a tempering process.

The needlelike martensitic structure forms in the working zone during local heating above the said temperatures and during subsequent rapid cooling. It is distinguished by a high degree of hardness and by brittleness. During operation, in such zones formation of cracks may occur in the strengthened zones. In addition, such operation leads to overheating of large masses of metal where the phase transitions will occur, which in turn will lead to increasing the values of the residual stresses, which also contribute to faster formation of cracks.

Heating below the said temperature limit will lead to essentially no transformation of the metal structure in the zone of thermal action, and also will have no effect on the hardness of the metal of the wheel pair. The service life or service performance will not be increased significantly in this case.

Yet another important characteristic that determines the service life or service performance of the railway wheelsets that have been locally hardened is the depth of the hardened region in which the martensitic structure will be present. A small depth of up to approximately 0.5 mm does not make it possible to achieve the inventive effect, because it will be worn off fairly quickly.

The proposed local hardening of the wheelsets in the premature wear zone can be undertaken after any restoration of the geometry of the profile of the wheelset during the process of repair.

Example for the implementation of the inventive method:

The proposed method was implemented using a group of 12 new wheelsets. The chemical composition and the mechanical properties of the metal of the wheels are characterized by the following parameters.

Chemical composition: C=0.60; Si=0.60; Mn=0.90; P=0.035; S=0.030

Mechanical properties:

Breaking strength G_(b)=980.66 MPa; unit elongation δ=10% and ψ=15%; Brinell hardness HB=280; flexural impact strength of samples with U-cutout KCU=34.32 Nm/cm².

The group of wheelsets was locally strengthened using the inventive method. On the apparatus described above, the wheelsets were heated in the region of the premature wear zone, in the above-described manner. The metal of the said zone was heated to the temperature of 1080° C. Thermal tempering at a temperature of 600° C. was performed on the same apparatus in a similar manner to eliminate residual stresses and to improve the structure. As a consequence of the operations performed, the hardness of the metal in the heat-treated annular zone (premature wear zone) was 580 HB. The depth of local hardening of the heat-treated sections was 3.8 mm (determined using microsection samples). The original hardness of the rail was 320 HB.

The 12 wheelsets thus treated were attached to the cars of a test railway train. The cars were loaded such that the load on the axle was 27 tons, which was nearly twice the standard load (15 tons). The railway train traveled over a 149,000 km railway test circuit during the course of the test run. The wheelsets were analyzed during the process of the test run. At the same travel distance, the wear in the normal wheelsets was 3.5-4 mm, while the wear in the strengthened wheelsets was 1.25-1.45 mm, which is to say lower wear by a factor of approximately 2.5.

According to the invention, the method for local strengthening of railway wheelsets makes it possible to increase the time between overhauls for wheelset operation by approximately 2.5-fold, specifically by reducing metal wear at the points of contact between the wheel rims and the adjacent rolling zones of the wheelsets (premature wear zone) with the wear of the railway rails remaining unchanged. The proposed method can be used equally well for the treatment of new and repaired wheelsets, which are used in railway vehicles, trams, and subway trains, and therefore can increase the time between overhauls in their operation. 

1. Method for local hardening, in particular of railway wheels, wherein each wheel is implemented in the form of a cylindrical working part having a rolling surface, and a rim connected to the cylindrical working part by a radius transition, comprising the metal hardening of the wheel by conduction of current through rotating contact rollers, which are pressed under pressure onto the surface to be treated, wherein the heating and hardening of the surface is performed in the form of one or more strips, characterized in that the heating for the hardening is carried out in a first and/or a second annular section, wherein the boundary of the first annular section begins at the connecting line of the radius transition at the beginning of the rolling surface and the boundary of the second annular section begins at the connecting line of the radius transition at the adjacent rim face, wherein the local heating in each of the said sections is carried out in the form of one or more annular strips, wherein the heating temperature is approximately 60-280° C. higher than the temperature of the phase transition Ac₃.
 2. Method according to claim 1, wherein the first annular section extends over an area of approximately 30 mm across the width of the rolling surface.
 3. Method according to claim 1, wherein the second annular section extends over an area of up to approximately 35 mm across the width of the rim surface.
 4. Method according to claim 1, wherein the distances between the zones of the heat action of each annular strip are at least approximately 2-4 mm from each side of the heating zone.
 5. Method according to claim 1, wherein the rotational speed of the wheel during heating is approximately 3.5-15 rotations per hour.
 6. Method according to claim 1, wherein the strips of heating are continuous or discontinuous annular sections. 